1. Field of the Invention
This disclosure relates in general to exhaust nozzles for upper surface blown aircraft and more particularly to an articulated dee nozzle for use in an upper surface blown aircraft which can vary its exit geometry.
2. Description of the Prior Art
Various types of short takeoff and landing (STOL) aircraft are in use. In general these aircraft include some means or form for augmenting or entirely replacing the aerodynamic lift of the wings during low speed operation occurring during takeoff and landing.
One type of aircraft that includes a method and apparatus for augmenting the aerodynamic lift created by its wings during low speed operation is an upper surface blown aircraft. An upper surface blown aircraft utilizes jet engines mounted so that the jet exhaust occurs above the wings rather than below the wings, as in conventional aircraft. The upper surface exhaust, during low speed operation, is "turned" downwardly over extended flaps located at the rear of the wing surface. The turning of the exhaust occurs without external mechanical means, in accordance with the well known Coanda effect. The downwardly directed exhaust provides the desired lift augmentation during low speed operation, particularly during takeoff and landing.
One of the problems with upper surface blowing aircraft relates to the cross-sectional configuration of the jet exhaust. A standard, relatively thick, jet exhaust will not follow the curve created by the extended flap. If the jet exhaust is vertically thick, it will separate from the wing and flap and not give the lift desired. A thick jet exhaust is characteristic of efficient cruise operation.
Conventional jet aircraft using under the wing engines obtain satisfactory performance during cruise and takeoff using a fixed exhaust nozzle geometry. Problems are encountered, however, in upper surface blown STOL aircraft using a Coanda flap system (a flap system that utilizes the Coanda effect) to obtain a powered lift that augments conventional aerodynamic lift. Effective Coanda flow turning can only occur if the thickness of the nozzle flow is limited to a certin percentage of the radius of curvature of the flap. If this limit is not met, the negative pressure naturally occurring on the wing upper surface side of the exhaust will be inadequate to turn the exhaust over the flap. Since flap size limits generally restrict the radius of flap curvature, the maximum exhaust flow thickness is correspondingly limited.
One way of attacking the above problem is to direct the exhaust flow downwardly, at a high angle relative to the horizontal plane by deflection or nozzle inclination. If a fixed nozzle exit area was sized for the STOL mode, the effective area of the nozzle would be too large at cruise resulting in engine nozzle mismatch and/or poor performance. Conversely, if the nozzle exit area is sized for cruise mode to overcome this disadvantage, the takeoff thrust would be below a tolerable level. A fixed high aspect ratio nozzle can be utilized to obtain the desired spreading, and resultant thinning, of the jet exhaust. However, this method results in cruise mode disadvantages. A wide nozzle, suitable for takeoff and landing has a reduced thrust efficiency, a high cruise drag and high weight. These disadvantages result in increased fuel consumption, and an equivalent loss in range.